1. Field of the Invention
The invention is related to vehicle control, and more specifically, to systems and methods for precisely and quickly pointing a vehicle in a desired direction.
2. Description of the Related Art
Turning a machine, such as a vehicle, or a portion thereof, quickly to point in a desired direction can be difficult. Turning precisely and quickly can be a challenge for flying and/or hovering vehicles, such as, for example, a helicopter or other rotorcraft (e.g., a tilt-rotor aircraft, a vertical/short takeoff and landing (VSTOL) aircraft), whether it is piloted or an unmanned aerial vehicle (UAV). Typically, internal and external effects, such as moments of inertia, angular momentum, structural limits, aerodynamic loads, and control surface actuator authority and rate limits must be dealt with.
For military aircraft, it may be especially important to have a capability to point a vehicle quickly in a desired direction, for example, to acquire, track, and fire upon an enemy target in advance of the enemy firing upon the aircraft. In this regard, the United States Army has established a performance specification entitled “AERONAUTICAL DESIGN STANDARD (ADS-33E-PRF) HANDLING QUALITIES REQUIREMENTS FOR MILITARY ROTORCRAFT,” that includes the following requirement for flight in Good Visual Environment (GVE) conditions:
“3.11.17 Turn to Target (Handling Qualities Requirement)
From a stable hover at an altitude of less than 20 ft complete a 180 deg turn. Turns must be completed in both directions. Final rotorcraft heading must be achieved within 5 seconds of initiating the turn within +/−3 degrees of the target.”
This task may be difficult with state-of-the-art technology because it is typically difficult for a pilot to time his or her directional control inputs precisely enough to get the aircraft to stop at the intended terminal heading without significant heading overshoot and time consuming heading backup toward the target heading. The heading overshoot and resulting heading backup problems are a result of aircraft yaw acceleration and jerk (time derivative of acceleration) limitations that prevent the pilot from instantaneously arresting the yaw rate developed during the turn.
In single rotor helicopters, these yaw acceleration and jerk limitations are imposed by the combined effect of tail rotor gearbox torque limits, tailrotor collective pitch actuator authority and rate limits, and limitations on allowable yaw acceleration due to inertial structural load limitations on attachment points for external stores.
Tandem rotor helicopters and tiltrotor aircraft face similar limits on allowable yaw acceleration and jerk due to rotor flapping induced blade and hub loads and differential cyclic pitch actuator authority and rate limits. Thus, the rapid turn to target task is difficult in virtually all vehicles with Vertical Takeoff Or Landing (VTOL) capabilities.
Meeting this requirement with existing rotorcraft control systems and configurations would rely heavily on pilot skill and training. For example, a pilot would need to anticipate, while turning a rotorcraft, the heading (azimuth) that the rotorcraft would achieve at the end of the turn once input force is released from the controls (e.g., rudder pedals), largely relying on experience and “feel” of the rotorcraft's handling characteristics, as well as mentally factoring in environmental conditions such as wind, altitude, etc. This results in an undesirably high pilot workload, and meeting the requirement using existing rotorcraft control systems and configurations could be impossible, or very difficult. In addition, there is a possibility of a pilot inadvertently exceeding the mechanical limits of a rotorcraft tail rotor gearbox and/or airframe while attempting to turn to a target quickly.
One possible solution is to build a rotorcraft with a large tail rotor, such as was proposed to be included in the Comanche helicopter program. However, such a solution may require an excessive cost, as well as design tradeoffs, such as increased weight and reduced agility.
This disclosure is directed toward overcoming one or more problems or disadvantages associated with the prior art.